1. Field of the Invention
The invention relates to a gain controller that is capable of controlling the gain of one or more amplifiers in a receiver.
The receive chain of a conventional receiver includes amplifiers for amplifying the received signal. In this way, the amplitude of the received signal can be altered so that it is sufficiently large for the signal to be successfully decoded but not so large that the signal is subject to clipping.
2. Description of Related Art
Many receivers include variable amplifiers. The gain of these amplifiers can be adjusted to compensate for variations in signal strength. Thus, when the received signal has a relatively large amplitude the gain of the amplifiers can be adjusted to be relatively low and when the received signal has a relatively small amplitude the gain of the amplifiers can be adjusted to be relatively high. The gain of the amplifiers is typically controlled by a gain controller, implemented in hardware or software, which adjusts the gain of the amplifiers in dependence on a measured signal strength of the received signal. Problems can arise if a loud interferer is being received when the strength of the received signal is measured to set the gain of the variable amplifier, as this can result in the gain being set too low. Loud interferers can cause errors of up to 9 or 12 dB. One method of addressing this problem is to operate the gain control as a free running process in which the gain control continuously measures the received signal strength and adjusts the gain of the variable amplifiers when necessary.
A disadvantage with the continuous gain control method described above is that using continuous gain control can tend to lead to “hunting”, in which the amplifier gain continuously oscillates about the correct gain. This problem is particularly acute in conventional gain controllers which typically make individual gain adjustment as high as 6 dB. A 6 dB error is sufficient to significantly reduce the quality of the decoding achieved by a receiver such as a modem. One solution to this problem would be to limit the length of time for which the continuous gain control is operational. However, this is problematic if a loud interferer is present during the limited length of time for which the gain is controlled continuously. A loud interferer can cause the gain controller to repeatedly reduce the gain to such an extent that when the interferer has stopped transmitting the gain is too low for the receiver to be able to correctly decode the wanted signal. These errors must be compensated for or higher order modulation schemes will not work.
Therefore, there is a need for an improved gain controller and method of gain control for a receiver.